Estimating lithologic facies in argillaceous and carbonate-rich mudrocks using x-ray fluorescence measurements and multivariate statistics
Argillaceous and carbonate-rich mudrocks are difficult to characterize mineralogically because of their fine grain size and complicated mineral assemblages and mineral formulas. Portable X-ray fluorescence (pXRF) allows for rapid and high-resolution measurement of 30 major and trace elemental concentrations at a spot size of approximately 1cm. We report a multivariate statistical approach to convert elemental concentrations into relevant lithofacies from geological core. With this approach we map lithofacies across a core and use correlations between lithofacies and hydrocarbon production-orientated parameters including total organic carbon, pore network distribution and permeability, and mechanical properties to better constrain properties of unconventional reservoirs. Correlations between core characterizations and well logs allows this approach to be scaled to the well and inter-well scale. Briefly, principal component analysis (PCA) constructs new principal components from the variance in the measured pXRF data sets. It minimizes the number of components necessary to describe the elemental variance in a dataset, making it easier to focus on the primary variations in the geologic core. PCA results are compared to a limited set of mineralogical modal abundances measured using x-ray diffraction (XRD) to convert the large number of pXRF data points to lithological assemblages. Here, we report a total of 1406 pXRF analyses and 44 x-ray diffraction (XRD) analyses from the Robert Todd core of western Louisiana, which covers 250’ of the Cretaceous Austin Chalk and 18’ of the upper Tuscaloosa Shale.
Three lithofacies are calculated from XRD based on percentages of total carbonate: chalk (>90%), marly chalk (80-90%), and chalky marl (70-80%). Considering the major elements, principal component 1 is dominated by Ca and Mn (carbonates) and Si, Al, K, Mg, and Fe (clay minerals, quartz and feldspars). pXRF elemental results are converted into lithofacies defined by the XRD results by comparing the statistical range (median, 25th and 75th percentile) of principal component one for each lithofacies. Using this approach we assign a lithofacies for all 1406 pXRF analyses. Large scale lithologic trends are readily illustrated using this appraoch. For example, there is an increase in carbonate content from the bottom of the Austin Chalk and the underlying argillaceous Tuscaloosa Shale to a depth of approximately 13050’, where nearly pure carbonate chalks are deposited. Alternating sequences of chalk to chalky marls are preserved throughout the length of the core, with alternations occurring on scales of less than 2’. These sequences are transitional rather than episodic and reflect alternating contributions of carbonate and clay minerals.
AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019